Lawson.] 



Malignite. 



347 



tergrowth, as will be described more particularly below. In general, 

 it seems to be true that the nepheline crystallized after the a;gerine- 

 augite, and for the most part before the orthoclase, but that a sub- 

 ordinate portion of it crystallized simultaneously with the latter. 



The nepheline is generally charged with decomposition prod- 

 ucts, but remains sufficiently intact to reveal its optical character. 

 Numerous sections were observed cut approximately normal to c, 

 which yielded in convergent light the uniaxial interference figure. 

 By the aid of the VjO. mica plate, the mineral was proven to be 

 negative. The index of the refraction is low, and the double refrac- 

 tion is weak. Cleavages are occasionally detected, and to these the 

 extinction is parallel. Slender colorless microlites (apatite) are spar- 

 ingly present as inclusions. The decomposition products are either 

 fibrous and polarize in brilliant colors with parallel extinction, or 

 they are cloudy kaolin-like aggregates. The development of these 

 secondary products seems to have induced a molecular tension in 

 the undecomposed nepheline, since between crossed nicols it gen- 

 erally manifests an undulatory extinction. 



When treated with dilute hydrochloric acid, the surface of the 

 thin sections of the nepheline gelatinizes and stains readily. If, prior 

 to staining, the solution be allowed to evaporate, an abundant deposit 

 of cubes of sodium chloride is obtained. These microchemical re- 

 actions were repeated several times on different slides with uniform 

 results, and, in some instances, grains were so treated which had 

 previously yielded the uniaxial interference figure. The character 

 of the mineral as nepheline is thus established beyond question. 



An interesting feature of the rock is the micropegmatitic inter- 

 growth of the nepheline and orthoclase. This is observable in a 

 limited number of areas which appear as somewhat cloudy spots in 

 the midst of the fresh pellucid orthoclase. Under high powers of 

 the microscope, they present a curious vermicular appearance, re- 

 sembling, if the homely simile may be used, a layer of vermicelli in 

 a plate of clear soup. The vermicular rods are usually very slender 

 and long. They are disposed either in a parallel or slightly diver- 

 gent arrangement, or present a curiously contorted appearance. 

 Sometimes they have a comb-like appearance, and are arranged as 

 a series of club-like bodies nearly normal to an axis composed of a 



